1. Field of the Invention
The present invention relates to an optical disc drive, and more particularly, to a method of forming volt level control focus when an optical disc drive scribes a label on a label side of an optical disc.
2. Description of the Prior Art
A light-scribe disc is formed by putting a layer of paint on a label side of the disc, and utilizing a laser beam from an optical pickup head to focus on the label side such that the heat will cause changes on the label side to form a label pattern. Insufficient laser beam heat will not be able to cause effective change and will therefore smear the label pattern. This means that the optical pickup head is required to move with the rotating optical disc to focus on the label side, and thus the laser heat can be concentrated to derive a clear label pattern. However, the paint of the label side is not distributed as evenly as a data side of the optical disc, and the reflective characteristic of the label side is also not as good as the data side, so an effective focus error signal cannot be generated to focus on the label side.
Please refer to FIG. 1 and FIG. 2 simultaneously. FIG. 1 is a prior art method of controlling focus of a label side of a disc. FIG. 2 is a label side focus volt level of a prior art. As shown in FIG. 1, a conventional optical pickup head 1 drives a lens 3 to move upwards and downwards via an electromagnetic force generated by adjusting a voltage of an actuator 2, and projects a laser beam from the optical pickup head 1 upon a label side 5 of a rotating disc 4. The label side 5 thereby reflects the laser beam back to the optical pickup head 1 and the laser beam finally projects to the photodetector 6. The photodetector 6 includes four light receiving parts A, B, C, D, which are evenly divided for receiving reflected light in different zones, respectively, and the photodetector 6 thereby converts the received light into electronic signals of corresponding magnitude. An amplifier 7 adds and amplifies the electronic signals in the light receiving parts A, B, C, D to form a side beam added signal (SBAD) of a corresponding magnitude h. During the focusing process, the lens 3 moves upwards and downwards by a predetermined distance S. When the focus point gets close to the label side 5, the derived SBAD increases gradually, and when the focus point is exactly on the label side 5, the reflected light is at its peak and the SBAD has a maximum value. When the focus point is over the label side 5, the derived SBAD will decrease with a distance between the focus point and the label side 5.
As shown in FIG. 2, the label side 5 of the light-scribe disc 4 is divided into eight radial zones 9 via four hundred spokes 8 with fixed angles, each zone having a group containing 50 spokes. When the laser beam of the optical pickup head 1 is projected on the rotating disc 4 at a fixed radial 10, SBADs are recorded during the focusing process of moving the optical pickup head 1 by a predetermined distance S via a volt at each zone 9. Due to a limitation of the interval of zone 9, the limited moving distance S is set by the focus point being over the label side 5 of the disc 4. Therefore, during the focusing process of each zone 9, a maximum SBAD of the focus point over the label side 4 can be derived. By referring to the volts and the spokes 8 corresponding to the maximum SBADs of the eight zones 9, a focus volt level curve 11 can be approximated. By utilizing the focus volt level curve 11, a focus volt corresponding to the spoke 8 where the optical pickup head 1 is located can be derived. Although the derived focus volt is different from the actual focus volt, the lens 3 can be moved to approximately focus on the label side 5 without utilizing the focus error signal to scribe label patterns.
Please refer to FIG. 1 again. When some warps or deformation occurs, such as the dotted line 4′ on the disc 4, during the process of focusing of each zone 9 with the predetermined distance S, the focus point cannot pass over a deformed label side 5′. In particular, when more zones are divided, the interval between zones grows smaller and the predetermined distance S becomes smaller, so the derived maximum SBAD does not indicate that the focus point is on the label side 5′, leading to a larger error upon the approximated focus volt level curve 11. This error will make the focus point of scribing label patterns away from the label side 5′ therefore resulting in a blurred label pattern, and degrading clarity of the label, even to a point of being unable to scribe the label pattern. Therefore, the control method of conventional focusing on a label side of a disc still has many problems to be solved.